1. Field of the Invention
The present invention relates to a heat-resistant magnesium alloy and, more particularly, to a heat-resistant magnesium alloy with excellent castability.
2. Description of Related Art
Recently, there has been increased need for the weight reduction of industrial materials, and as the lightweight material, magnesium alloys lighter than aluminum alloys have drawn researchers attention. Magnesium alloys are the lightest among commercial metals, and now being used as the material for automobiles as well as air planes. For example, magnesium alloys have been already used as the material for road wheels and engine head covers of automobiles.
In addition, there has been great need for the weight reduction of all automobile parts, and the application range of magnesium alloys tends to be enlarging further. For example, it has been proposed to apply magnesium alloys even to structural parts such as engine blocks or the like, and functional parts such as pistons or the like, of which the temperatures tend to be elevated. If the pistons, for example, which have conventionally been made of aluminum alloys are made of magnesium alloys, the weight thereof could be reduced, and the inertia d force or the like could be also reduced, thereby enabling weight reduction of other parts, too.
Magnesium alloy products are generally produced by casting procedure including die-casting. So, to enlarge the application range of magnesium alloys, the improvement of castability thereof is needed. Furthermore, to reduce the mass production costs, the improvement of yield rate is also needed by restraining and preventing the occurrence of casting cracks (not cracking or not tears) or any casting defects.
Under these circumstances, various kinds of magnesium alloys have been developed. Japanese Unexamined Patent Publication No. Sho 61(1986)-3863 discloses Mg--Al--Si--Mn alloys and Japanese Unexamined Patent Publication Nos. Hei 6(1994)-25791 and Hei 7(1995)-18364 disclose Mg--Zn--Ca alloys, which exhibit improved heat resistance superior to that of conventional Mg--Al--Zn--Mn alloys.
In addition, various kinds of Mg--RE--Zn alloys (RE: rare earth element(s)), which are different from the above-described alloys, have been also developed. Mg--RE--Zn alloys have been reported to exhibit good heat resistance and castability superior to those of the Mg--Zn--Ca alloys, and are disclosed in British patent publications Nos. P637040, P1378281, Japanese Unexamined Patent Publication No. Hei 9(1997)-256099 and Publication of translation of International patent application No. Hei 10-513225 (WO96/24701), etc. British patent publications Nos. P-637040 and P1378281 disclose that the addition of 0.1 to 0.9% by weight of zirconium as a grain-refining element is preferable.
Furthermore, Japanese Unexamined Patent Publication No. Hei 9(1997)-256099 discloses that 0.5 to 3% by weight of calcium can restrain the occurrence of casting cracks in the conventional Mg--Zn--Ca alloys.
Upon further investigations and researches on the magnesium alloys disclosed in Japanese Unexamined Patent Publication No. Hei 9(1997)-256099, the present inventors have clarified that in the case of magnesium alloys, when casting strain is small during casting, no crack occurs, but when casting strain becomes greater during casting, cracks may occur. The reason of the occurrence of casting strain can be considered as follows.
During casting, thermal contraction occurs due to solidification shrinkage and cooling of cast products. This thermal contraction causes the occurrence of casting strain. Provided that the cooling process is identical, the amount of casting strain depends only on the configuration of castings. For example, at corners or the like, under strong constraint, the amount of casting strain increases. However, actually, the cooling process varies from place to place due to the variation of thickness and the casting design in addition to the configuration of casting. Consequently, in low cooling rate parts, a larger casting strain may occur, and consequently cracks may occur, as compared to high cooling rate peripheral parts. Thus, casting cracks may occur in ribs, bosses and corners where the thickness greatly varies.
Publication of translation of international patent application No. Hei 10-513225 (WO96/24701) discloses that calcium operates as a castability modifier, but does not disclose any specific embodiment. This publication merely discloses that the calcium content ranges from 0 to 1% by weight. Furthermore, the present inventors have recognized that calcium makes ignition-resistant molten alloys, and consequently serves to improve the work efficiency in melting and casting operations, but may cause defects such as casting cracks, resulting in cast products being difficult to form.
Upon these investigations by the present inventors, no conventional technique of severely adjusting the calcium content to highly effect both the heat resistance and castability of Mg--RE--Zn alloys has been found yet.